System and method for controlling combined radio signals

ABSTRACT

A method for controlling a combined waveform, representing a combination of at least two signals having orthogonal frequency multiplexed signal components, comprising: receiving information defining the at least two signals; transforming the information defining each signal to a representation having orthogonal frequency multiplexed signal components, such that at least one signal has at least two alternate representations of the same information, and combining the transformed information using the at least two alternate representations, in at least two different ways, to define respectively different combinations; analyzing the respectively different combinations with respect to at least one criterion; and outputting a respective combined waveform or information defining the waveform, representing a selected combination of the transformed information from each of the at least two signals selected based on the analysis.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.14/553,580, filed Nov. 25, 2014, now U.S. Pat. No. 9,401,923, issuedJul. 26, 2016, and is a Continuation of U.S. U.S. patent applicationSer. No. 14/954,326, filed Nov. 30, 2015, now pending, which is aContinuation of U.S. U.S. patent application Ser. No. 14/553,631, filedNov. 25, 2014, now U.S. Pat. No. 9,203,654, issued Dec. 1, 2015, each ofwhich claims benefit of priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/909,252, filed Nov. 26, 2013, theentirety of which are expressly incorporated herein by reference.

This application is related to PCT/US14/67426, WO 2015/081107.

FIELD OF THE INVENTION

This invention relates to the field of wireless communications ofradio-frequency signals. More specifically, it relates to controlling acombined signal, for example to reduce its peak to average power ratioor an inferred error at a receiver.

BACKGROUND OF THE INVENTION

A common signal format for mobile wireless communications is orthogonalfrequency-domain multiplexing (OFDM) (see, for example,en.wikipedia.org/Orthogonal_frequency-division_multiplexing), andclosely related formats such as orthogonal frequency-domain multipleaccess (OFDMA). For a signal conveyed on an OFDM channel, this ischaracterized in the frequency domain by a bundle of narrow adjacentsubchannels, and in the time domain by a relatively slow series of OFDMsymbols each with a time T, each separated by a guard interval ΔT (seeFIG. 1B). Within the guard interval before each symbol is a cyclicprefix (CP), comprised of the same signal in the symbol period,cyclically shifted in time. This CP is designed to reduce thesensitivity of the received signal to precise time synchronization inthe presence of multipath, i.e., radio-frequency signals reflecting fromlarge objects in the terrain such as tall buildings, hills, etc. If agiven symbol is received with a slight time delay (less than ΔT), itwill still be received without error. In addition to the data symbolsassociated with the OFDM “payload”, there is also typically a “preamble”signal that establishes timing and other standards. The preamble mayhave its own CP, not shown in FIG. 1B.

In addition to the preamble, a set of pilot symbols (also calledtraining symbols) are typically interleaved (in time and frequency)among the data symbols in the payload. These pilot symbols are usedtogether with the preamble for further refinement of timing, channelestimation, and signal equalization at the receiver. The particularplacement of pilot symbols in time and frequency within the payload maydiffer among various OFDM standard protocols. A typical example of theplacement of pilot symbols in the time-frequency resource grid is shownin FIG. 2 for a protocol known as “Long-Term Evolution” (LTE). (See, forexample, www.mathworks.com/help/lte/ug/channel-estimation.html forfurther information.) Here pilot symbols are located at four differentfrequencies, with a pattern that repeats every eight symbol periods.This enables the receiver to obtain information on time-varying channelestimation across the entire resource grid, using interpolation of thevarious pilot symbols.

In OFDM, the sub-carrier frequencies are chosen so that the sub-carriersare orthogonal to each other, meaning that cross-talk between thesub-channels is eliminated and inter-sub-carrier guard bands are notrequired. This greatly simplifies the design of both the transmitter andthe receiver; unlike conventional FDM, a separate filter for eachsub-channel is not required. The orthogonality requires that thesub-carrier spacing is Δf=k/(T_(U)) Hertz, where T_(U) seconds is theuseful symbol duration (the receiver side window size), and k is apositive integer, typically equal to 1. Therefore, with N sub-carriers,the total passband bandwidth will be B≈N·Δf (Hz). The orthogonality alsoallows high spectral efficiency, with a total symbol rate near theNyquist rate. Almost the whole available frequency band can be utilized.OFDM generally has a nearly “white” spectrum, giving it benignelectromagnetic interference properties with respect to other co-channelusers.

When two OFDM signals are combined, the result is in general anon-orthogonal signal. While a receiver limited to the band of a singleOFDM signal would be generally unaffected by the out-of-channel signals,when such signals pass through a common power amplifier, there is aninteraction, due to the inherent non-linearities of the analog systemcomponents.

OFDM requires very accurate frequency synchronization between thereceiver and the transmitter; with frequency deviation the sub-carrierswill no longer be orthogonal, causing inter-carrier interference (ICI),i.e. cross-talk between the sub-carriers. Frequency offsets aretypically caused by mismatched transmitter and receiver oscillators, orby Doppler shift due to movement. While Doppler shift alone may becompensated for by the receiver, the situation is worsened when combinedwith multipath, as reflections will appear at various frequency offsets,which is much harder to correct.

The orthogonality allows for efficient modulator and demodulatorimplementation using the fast Fourier transform (FFT) algorithm on thereceiver side, and inverse FFT (IFFT) on the sender side. While the FFTalgorithm is relatively efficient, it has modest computationalcomplexity which may be a limiting factor.

One key principle of OFDM is that since low symbol rate modulationschemes (i.e. where the symbols are relatively long compared to thechannel time characteristics) suffer less from intersymbol interferencecaused by multipath propagation, it is advantageous to transmit a numberof low-rate streams in parallel instead of a single high-rate stream.Since the duration of each symbol is long, it is feasible to insert aguard interval between the OFDM symbols, thus eliminating theintersymbol interference. The guard interval also eliminates the needfor a pulse-shaping filter, and it reduces the sensitivity to timesynchronization problems.

The cyclic prefix, which is transmitted during the guard interval,consists of the end of the OFDM symbol copied into the guard interval,and the guard interval is transmitted followed by the OFDM symbol. Thereason that the guard interval consists of a copy of the end of the OFDMsymbol is so that the receiver will integrate over an integer number ofsinusoid cycles for each of the multipaths when it performs OFDMdemodulation with the FFT.

The effects of frequency-selective channel conditions, for examplefading caused by multipath propagation, can be considered as constant(flat) over an OFDM sub-channel if the sub-channel is sufficientlynarrow-banded, i.e. if the number of sub-channels is sufficiently large.This makes equalization far simpler at the receiver in OFDM incomparison to conventional single-carrier modulation. The equalizer onlyhas to multiply each detected sub-carrier (each Fourier coefficient) bya constant complex number, or a rarely changed value. Therefore,receivers are generally tolerant of such modifications of the signal,without requiring that explicit information be transmitted.

OFDM is invariably used in conjunction with channel coding (forwarderror correction), and almost always uses frequency and/or timeinterleaving. Frequency (subcarrier) interleaving increases resistanceto frequency-selective channel conditions such as fading. For example,when a part of the channel bandwidth is faded, frequency interleavingensures that the bit errors that would result from those subcarriers inthe faded part of the bandwidth are spread out in the bit-stream ratherthan being concentrated. Similarly, time interleaving ensures that bitsthat are originally close together in the bit-stream are transmitted farapart in time, thus mitigating against severe fading as would happenwhen travelling at high speed. Therefore, similarly to equalization perse, a receiver is typically tolerant to some degree of modifications ofthis type, without increasing the resulting error rate.

The OFDM signal is generated from the digital baseband data by aninverse (fast) Fourier transform (IFFT), which is computationallycomplex, and as will be discussed below, generates a resulting signalhaving a relatively high peak to average power ratio (PAPR) for a setincluding a full range of symbols. This high PAPR, in turn generallyleads to increased acquisition costs and operating costs for the poweramplifier (PA), and typically a larger non-linear distortion as comparedto systems designed for signals having a lower PAPR. This non-linearityleads, among other things, to clipping distortion and intermodulation(IM) distortion, which have the effect of dissipating power, causingout-of-band interference, and possibly causing in-band interference witha corresponding increase in bit error rate (BER) at a receiver.

In a traditional type OFDM transmitter, a signal generator performserror correction encoding, interleaving, and symbol mapping on an inputinformation bit sequence to produce transmission symbols. Thetransmission symbols are subjected to serial-to-parallel conversion atthe serial-to-parallel (S/P) converter and converted into multipleparallel signal sequences. The S/P converted signal is subjected toinverse fast Fourier transform at the IFFT unit. The signal is furthersubjected to parallel-to-serial conversion at the parallel-to-serial(P/S) converter, and converted into a signal sequence. Then, guardintervals are added by the guard interval (GI) adding unit. Theformatted signal is then up-converted to a radio frequency, amplified atthe power amplifier, and finally transmitted as an OFDM signal by aradio antenna.

On the other hand, in a traditional type OFDM receiver, the radiofrequency signal is down-converted to baseband or an intermediatefrequency, and the guard interval is removed from the received signal atthe guard interval removing unit. Then, the received signal is subjectedto serial-to-parallel conversion at S/P converter, fast Fouriertransform at the fast Fourier transform (FFT) unit, andparallel-to-serial conversion at P/S converter. Then, the decoded bitsequence is output.

It is conventional for each OFDM channel to have its own transmit chain,ending in a power amplifier (PA) and an antenna element. However, insome cases, one may wish to transmit two or more separate OFDM channelsusing the same PA and antenna, as shown in FIG. 3. This is sometimescalled “carrier aggregation”. This may permit a system with additionalcommunications bandwidth on a limited number of base-station towers.Given the drive for both additional users and additional data rate, thisis highly desirable. The two channels may be combined at an intermediatefrequency using a two-stage up-conversion process as shown in FIG. 3.Although amplification of real baseband signals is shown in FIG. 3, ingeneral one has complex two-phase signals with in-phase and quadratureup-conversion (not shown). FIG. 3 also does not show the boundarybetween digital and analog signals. The baseband signals are normallydigital, while the RF transmit signal is normally analog, withdigital-to-analog conversion somewhere between these stages.

Consider two similar channels, each with average power P₀ and maximuminstantaneous power P₁. This corresponds to a peak-to-average powerratio PAPR=P₁/P₀, usually expressed in dB as PAPR[dB]=10 log(P₀/P₀). Forthe combined signal, the average power is 2 P₀ (an increase of 3 dB),but the maximum instantaneous power can be as high as 4 P₀, an increaseof 6 dB. Thus, PAPR for the combined signal can increase by as much as 3dB. This maximum power will occur if the signals from the two channelshappen to have peaks which are in phase. This may be a rare transientoccurrence, but in general the linear dynamic range of all transmitcomponents must be designed for this possibility. Nonlinearities willcreate intermodulation products, which will degrade the signal and causeit to spread into undesirable regions of the spectrum. This, in turn,may require filtering, and in any case will likely reduce the powerefficiency of the system.

Components with required increases in linear dynamic range to handlethis higher PAPR include digital-to-analog converters, for example,which must have a larger number of effective bits to handle a largerdynamic range. But even more important is the power amplifier (PA),since the PA is generally the largest and most power-intensive componentin the transmitter. While it is sometimes possible to maintaincomponents with extra dynamic range that is used only a small fractionof the time, this is wasteful and inefficient, and to be avoided wherepossible. An amplifier with a larger dynamic range typically costs morethan one with a lower dynamic range, and often has a higher quiescentcurrent drain and lower efficiency for comparable inputs and outputs.

This problem of the peak-to-average power ratio (PAPR) is a well-knowngeneral problem in OFDM and related waveforms, since they areconstructed of multiple closely-spaced subchannels. There are a numberof classic strategies to reducing the PAPR, which are addressed in suchreview articles as “Directions and Recent Advances in PAPR ReductionMethods”, Hanna Bogucka, Proc. 2006 IEEE International Symposium onSignal Processing and Information Technology, pp. 821-827, incorporatedherein by reference. These PAPR reduction strategies include amplitudeclipping and filtering, coding, tone reservation, tone injection, activeconstellation extension, and multiple signal representation techniquessuch as partial transmit sequence (PTS), selective mapping (SLM), andinterleaving. These techniques can achieve significant PAPR reduction,but at the expense of transmit signal power increase, bit error rate(BER) increase, data rate loss, increase in computational complexity,and so on. Further, many of these techniques require the transmission ofadditional side-information (about the signal transformation) togetherwith the signal itself, in order that the received signal be properlydecoded. Such side-information reduces the generality of the technique,particularly for a technology where one would like simple mobilereceivers to receive signals from a variety of base-stationtransmitters. To the extent compatible, the techniques disclosed inBogucka, and otherwise known in the art, can be used in conjunction withthe techniques discussed herein-below.

Various efforts to solve the PAPR (Peak to Average Power Ratio) issue inan OFDM transmission scheme, include a frequency domain interleavingmethod, a clipping filtering method (See, for example, X. Li and L. J.Cimini, “Effects of Clipping and Filtering on the Performance of OFDM”,IEEE Commun. Lett., Vol. 2, No. 5, pp. 131-133, May, 1998), a partialtransmit sequence (PTS) method (See, for example, L. J Cimini and N. R.Sollenberger, “Peak-to-Average Power Ratio Reduction of an OFDM SignalUsing Partial Transmit Sequences”, IEEE Commun. Lett., Vol. 4, No. 3,pp. 86-88, March, 2000), and a cyclic shift sequence (CSS) method (See,for example, G. Hill and M. Faulkner, “Cyclic Shifting and TimeInversion of Partial Transmit Sequences to Reduce the Peak-to-AverageRatio in OFDM”, PIMRC 2000, Vol. 2, pp. 1256-1259, September 2000). Inaddition, to improve the receiving characteristic in OFDM transmissionwhen a non-linear transmission amplifier is used, a PTS method using aminimum clipping power loss scheme (MCPLS) is proposed to minimize thepower loss clipped by a transmission amplifier (See, for example, XiaLei, Youxi Tang, Shaoqian Li, “A Minimum Clipping Power Loss Scheme forMitigating the Clipping Noise in OFDM”, GLOBECOM 2003, IEEE, Vol. 1, pp.6-9, Dec. 2003). The MCPLS is also applicable to a cyclic shiftingsequence (CSS) method.

In a partial transmit sequence (PTS) scheme, an appropriate set of phaserotation values determined for the respective subcarriers in advance isselected from multiple sets, and the selected set of phase rotations isused to rotate the phase of each of the subcarriers before signalmodulation in order to reduce the peak to average power ratio (See, forexample, S. H. Muller and J. B. Huber, “A Novel Peak Power ReductionScheme for OFDM”, Proc. of PIMRC '97, pp. 1090-1094, 1997; and G. R.Hill, Faulkner, and J. Singh, “Deducing the Peak-to-Average Power Ratioin OFDM by Cyclically Shifting Partial Transmit Sequences”, ElectronicsLetters, Vol. 36, No. 6, 16, March, 2000).

When multiple radio signals with different carrier frequencies arecombined for transmission, this combined signal typically has anincreased PAPR, owing to the possibility of in-phase combining of peaks,requiring a larger power amplifier (PA) operating at low averageefficiency. As taught by U.S. Pat. No. 8,582,687 (J. D. Terry),expressly incorporated herein by reference in its entirety, the PAPR fordigital combinations of OFDM channels may be reduced by a Shift-and-AddAlgorithm (SAA): Storing the time-domain OFDM signals for a given symbolperiod in a memory buffer, carrying out cyclic time shifts to transformat least one OFDM signal, and adding the multiple OFDM signals to obtainat least two alternative combinations. In this way, one can select thetime-shift corresponding to reduced PAPR of the combined multi-channelsignal. This may be applied to signals either at baseband, or onupconverted signals. Several decibels reduction in PAPR can be obtainedwithout degrading system performance. No side information needs to betransmitted to the receiver, provided that the shifted signal can bedemodulated by the receiver without error. This is shown schematicallyin FIG. 4.

Some OFDM protocols may require a pilot symbol every symbol period,where the pilot symbol may be tracked at the receiver to recover phaseinformation (see FIG. 5). If the time-shift is performed on a given OFDMcarrier, according to such a protocol, during a specific symbol period,the pilot symbol will be subject to the same time-shift, so that thereceiver will automatically track these time-shifts from one symbolperiod to the next. However, as indicated in FIG. 2, typical modern OFDMprotocols incorporate a sparser distribution of pilot symbols, withinterpolation at the receiver to generate virtual pilot symbols(reference signals) for other locations. With such a protocol, anarbitrary time shift as implemented in the SAA may not be properlytracked, so that without side information, bit errors may be generatedat the receiver.

What is needed is a practical method and associated apparatus forreducing the PAPR of combined OFDM signals in a wide variety of modernOFDM protocols, in a way that does not degrade the received signal orrequire the transmission of side-information.

The following patents, each of which are expressly incorporated hereinby reference, relate to peak power ratio considerations: U.S. Pat. Nos.7,535,950; 7,499,496; 7,496,028; 7,467,338; 7,463,698; 7,443,904;7,376,202; 7,376,074; 7,349,817; 7,345,990; 7,342,978; 7,340,006;7,321,629; 7,315,580; 7,292,639; 7,002,904; 6,925,128; 7,535,950;7,499,496; 7,496,028; 7,467,338; 7,443,904; 7,376,074; 7,349,817;7,345,990; 7,342,978; 7,340,006; 7,339,884; 7,321,629; 7,315,580;7,301,891; 7,292,639; 7,002,904; 6,925,128; 5,302,914; 20100142475;20100124294; 20100002800; 20090303868; 20090238064; 20090147870;20090135949; 20090110034; 20090110033; 20090097579; 20090086848;20090080500; 20090074093; 20090067318; 20090060073; 20090060070;20090052577; 20090052561; 20090046702; 20090034407; 20090016464;20090011722; 20090003308; 20080310383; 20080298490; 20080285673;20080285432; 20080267312; 20080232235; 20080112496; 20080049602;20080008084; 20070291860; 20070223365; 20070217329; 20070189334;20070140367; 20070121483; 20070098094; 20070092017; 20070089015;20070076588; 20070019537; 20060268672; 20060247898; 20060245346;20060215732; 20060126748; 20060120269; 20060120268; 20060115010;20060098747; 20060078066; 20050270968; 20050265468; 20050238110;20050100108; 20050089116; and 20050089109.

The following patents, each of which is expressly incorporated herein byreference, relate to one or more topics in wireless radio-frequencycommunication systems: U.S. Pat. Nos. 8,130,867; 8,111,787; 8,204,141;7,646,700; 8,520,494; 20110135016; 20100008432; 20120039252;20130156125; 20130121432; 20120328045; 2013028294; 2012275393;20110280169; 2013001474; 20120093088; 2012224659; 20110261676;WO2009089753, WO2013015606; 20100098139; 20130114761; WO2010077118A2.

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SUMMARY OF THE INVENTION

The present invention extends and generalizes the prior art of Terry(U.S. Pat. No. 8,582,687) in the carrier aggregation of two or more OFDMsignals in different frequency bands. For a preferred embodiment,consider a first and a second OFDM signal to be combined andtransmitted, where in a given symbol period, the candidate signaltransformations of the first OFDM signal are restricted to those thatcan be demodulated by a receiver without side information (see FIG. 6).For example, the digital processor at the transmitter (the “transmitprocessor”) can use the pilot symbols previously transmitted for thefirst signal to interpolate the same virtual pilot symbols (referencesignals) that are also generated by the receiver. Then the transmitprocessor can select at least two versions of the first OFDM signal thatare compatible with the receive protocol. Each of these at least twoOFDM signal versions can be combined with the second OFDM signal at adifferent frequency (carrier aggregation) as per the prior art Terryalgorithm, and the PAPR (or other figure of merit) for the combinedsignal can be evaluated. In this way, the optimization can be restrictedto appropriate signal candidates, without wasting computationalresources on unacceptable alternatives.

This method can be further generalized beyond OFDM for any present orfuture communications system based on multiple subchannels, which mightbe labeled MSM (multiple subchannel multiplexing). Typically, suchsubchannels are orthogonal, that is, not subject to intersymbolinterference across channels, but this is not an absolute requirement,since in various instances, such interference can be resolved, and inany case when subject to significant Doppler shifts, true orthogonalitymay be lost. On the other hand, signals may be defined absent strictorthogonality, but be received as orthogonal signals, for example, dueto Doppler shifts.

The subchannels are typically frequency channels, such that thesubchannels represent a series of frequency assignments within a channelwhich occupies a range of frequencies. In some cases, however, thesubchannels may correspond to other assignments. For example, a typicalscheme for generating subchannels in an orthogonal frequency divisionmultiplexed (OFDM) signal is to modulate a signal by performing aninverse fast Fourier transform (IFFT) at the transmitter to generate theorthogonal frequency subcarriers, and perform a fast Fourier transform(FFT) at the receiver to demodulate the information from thesubcarriers. Typically, a sparse selection of the subcarriers over timeand over frequency communicate pilot signals, which permit calibrationof the receiver to account for channel conditions. One issue addressedby the present technology is that a time shift of the entire OFDM signalcan lead, under some conditions of the time shift and the frequencysubchannel placement of the pilot signal, to a failure of an ability toproperly receive the pilot signal, and therefore represents an invalidcombination of pilot frequency and cyclic shift. Therefore according toone aspect of the technology, a proposed cyclic shift of a multiplesubchannel modulated signal is tested against a model of the receiverand/or the channel conditions and receiver, to ensure compatibility withsuccessful receipt of the pilot signal(s) communicated within a symbolperiod.

The technology can be extended to non-frequency subchannel assignments,for example when the transform used a the transmitter is a differenttransform, for example, an inverse wavelet transform with acorresponding wavelet transform performed at the receiver. See, each ofwhich is expressly incorporated herein by reference in its entirety:

Rohit Bodhe et al., “Design Of Simulink Model For OFDM and Comparison ofFFT-OFDM and DWT-OFDM”, International Journal of Engineering Science andTechnology (BEST) Vol. 4 No. 05, May 2012 pp. 1914-1924; 1.Communication Systems, 4th edition, Simon Haykin, John Wiley and Sons,Inc.;

C. V. Bouwel, et. al, Wavelet Packet Based Multicarrier Modulation, IEEECommunications and Vehicular Technology, SCVT 200, pp. 131-138, 2000; B.G. Nagesh, H. Nikookar, Wavelet Based OFDM for Wireless Channels, IEEEVehicular Technology Conference, Vol. 1, pp. 688-691, 2001; LI Wiehua,et. al, Bi-orthogonal Wavelet Packet based Multicarrier modulation;

Haixia Zhang, et. al, Research of DFT-OFDM and DWT-OFDM on DifferentTransmission Scenarios. Proceeding of the second internationalconference on Information Technology for Application (ICITA 2004);

B. G. Negash and H. Nikookar, “Wavelet based OFDM for wirelesschannels,” Vehicular Technology Conference, 2001;

A. N. Akansu and L. Xueming, “A comparative performance evaluation ofDMT (OFDM) and DWMT (DSBMT) based DSL communications systems for singleand multitone interference,” Proceedings of the IEEE InternationalConference on Acoustics, Speech and Signal Processing, 1998;

Khaizuran Abdullah, “Performance of Fourier-Based and Wavelet-Based OFDMfor DVB-T Systems”, 2007 Australasian Telecommunication Networks andApplications Conference Dec. 2-5, 2007, Christchurch, New Zealand pp.475-479 (2007);

Rohit Bodhe et al, “Performance Comparison of FFT and DWT based OFDM andSelection of Mother Wavelet for OFDM” (IJCSIT) International Journal ofComputer Science and Information Technologies, Vol. 3 (3), 2012, pp.3993-3997.

Similarly to OFDM, any such system may employ at least one pilot signaltransmitted to the receiver, in order to calibrate the state of thecommunication channel. Since the channel state will vary with time andacross subchannels (particularly in a mobile system with multipath andinterference), the pilot signal may be interleaved among the signal datarepresentation present in the subchannels over time. Typically, a smallportion of the subchannels over time and over a range of differentfrequencies, will be allocated to transmitting pilot signals. This willpermit the receiver to track the changing communication channel tominimize the error rate of received data. An adaptive system may beprovided which alters the insertion of pilot signals interspersed in thedata communication dependent on actual error rates and channelconditions. Thus, in a channel where the error rate is low, fewer pilotsignals may be communicated, permitting higher peak data rates.Likewise, under different types of noise conditions, the pilot signals,which are provided to address changes in channel conditions, can betraded off against error correction signals, which address noiseconditions.

Furthermore, the communication protocol should allow the received datarepresentation to vary along at least one parameter (in addition toamplitude), in order to reflect the varying channel environment. A keyaspect is the recognition that the transmitted signal may alsoincorporate allowable variation in this at least one parameter. In orderto accurately determine the acceptable range of the allowable variation,the transmit processor may simulate how the receive processor for thegiven signal will make use of the current and prior pilot signals. Iftwo or more signals or bands are combined (carrier aggregation), thedegree of freedom associated with this allowed variation of the at leastone parameter may be exploited to optimize a separate fitness criterionor figure of merit, such as a peak-to-average power ratio (PAPR) or abit-error ratio (BER) that may vary with this degree of freedom. In thepreferred embodiment described below, the degree of freedom is cyclicshifting of the signal (which would correspond to variation in thephysical path length), but other transformations may also be possible,such as frequency shifting (emulating a Doppler shift), phase shifting,synthetic multipath (time delayed replica), and deviation fromorthogonality for subcarriers. Furthermore, the description of thepreferred embodiment in no way limits the scope of the invention. Inorder to identify appropriate signal transformation candidates, apreferred embodiment of the present invention builds on a set of digitalsignal processing techniques known in the prior art as “codebooktransmission” or “codebook pre-weighting” or “precoding”. Codebooktransmission derives its origin from cryptography. A codebook contains alookup table for coding and decoding; each word or phrase has one ormore strings which replace it. More recently, the term precoding hasbeen used in conjunction with closed loop beamforming techniques inmulti-antenna wireless communication systems, where channel stateinformation is sent to the transmitting device from the receiving deviceto optimize the transmission for the current state of the channel. See,for example, the Wikipedia entry on “Precoding”:en.wikipedia.org/wiki/Precoding.

It should not be understood here that the present method for RF carrieraggregation relies on closed-loop or multi-antenna systems, but ratherthat similar mathematical techniques are applied. However, thissimilarity also enables straightforward integration of efficient carrieraggregation with multi-antenna and closed-loop communication systems.

Codebook techniques can generate a lookup table of channel responsescorresponding to at least two different transformations of the pilotsymbols for specific OFDM protocols, and corresponding allowable channelresponses for those locations in the resource grid without any pilotsymbols. This is indicated in the generalized flowchart shown in FIG. 7.By allowable, we mean that process steps depicted in FIG. 8 and FIG. 9allow recovery of the OFDM data symbols with minimal or no degradationat the receiver beyond the effects due to the physical channel, and notrequiring any additional side information.

A preferred embodiment of the present system and method (shown in FIG.10 and FIG. 11) seeks to control the PAPR by storing the time-domainOFDM signals for a given symbol period in a memory buffer, and carryingout cyclic time shifts of at least one of the OFDM signals, in order toselect the time-shift corresponding to a desired PAPR of the combinedmulti-channel signal. In most cases, it would be desired to reduce thePAPR to a minimum, but this is not a limitation of the technique, andthe selected time-shift may be based on other criteria.

It is noted that each of the OFDM signals may be preprocessed inaccordance with known schemes, and thus each may have been themselvesprocessed to reduce an intrinsic PAPR, though preferably anypreprocessing of the signals is coordinated with the processing of thecombined signals to achieve an optimum cost and benefit. For example,where two separate signals are to be combined, each having a high PAPR,a resulting signal of reduced PAPR can be achieved if the peaks add outof phase, and thus cancel. Therefore, initial uncoordinated efforts tomodify the input OFDM signals may have limited benefit.

It is further noted that the present system seeks to combineindependently formatted OFDM signals, which are generally targeted todifferent receivers or sets of receivers, and these sets are typicallynot coordinated with each other. For example, in a cellular transceiversystem, a base station may serve hundreds or thousands of cell phones,each phone monitoring a single OFDM broadcast channel, with the basestation servicing multiple OFDM channels. It is particularly noted thateach set of OFDM subcarriers is orthogonal, but the separate OFDMsignals, and their subcarriers, are generally not orthogonal with eachother. The OFDM signals may be in channels which are adjacent ordisplaced, and therefore a relative phase change between OFDM signalscan occur during a single symbol period. Therefore, the PAPR must beconsidered over the entire symbol period.

Indeed, according to another embodiment of the method, it is not thePAPR of the signal which is analyzed for optimization, but rather aninferred error at the receiver. Thus, if the PAPR of the compositesignal is high for only a small portion of a symbol period, such thatthe PA distorts or clips the signal at that time, but at most othertimes the combined signals are well within specification, the result maybe an acceptable transmission which would likely result in a low errorprobability. Indeed, in some cases, the error probability may be lowerthan for signals with a lower absolute peak. Therefore, by employing amodel of a receiver, which itself may include margins for specificcommunication channel impairments to specific receivers, and Dopplershifts (which may be determined, for example by analyzing return pathcharacteristics), or over a range of possible variation, as part of thetransmitter signal processing path, better performance may be availablethan by simply minimizing the PAPR.

The receiver model seeks to implement the critical functions of anidealized receiver compliant with the communication protocol, as well asoptionally a channel conditions model or range of possible impairmentcondition models. In the case of an OFDM receiver, the received signalis demodulated, e.g., to baseband, and an FFT applied to separatesubbands into frequency bins. In some timeframes, and some subbands,pilot signals are inserted instead of data, according to a predeterminedprotocol. If a small number of pilot signals are to be extracted fromthe OFDM signal, a Goertzel Algorithm may also be used. The receiverknows where these pilot signals are to be found, and analyzes theseseparately for various distortions which indicate channel conditions.Since channel conditions change slowly with respect to data frames, thepilot transmission may be sparse, and some data frames may not includepilot signals. The pilot signals are typically spread into differentfrequency bins, to map the conditions across the entire channel. Theremaining frequency bins are then analyzed to extract the subband data.The pilot signal may be used to correct the demodulation of data fromthe information subbands, i.e., calibrate frequency bin boundaries inthe presence of Doppler shift and the like.

When the OFDM signal is cyclically shifted, this appears to the receiversimilar to a time shift (delay). Therefore, the cyclic shift ispermissible to the receiver within the range of its permissible changein time delay between symbols. The receiver model therefore maintains ina memory the prior states of the time shifts, which will control theacceptability of a successive change in time shift.

According to the model, if the various pilot signals are sufficientlycorrupted, the data cannot reliably be demodulated from the OFDM signal,and a packet retransmit, for example, is requested. In the receivermodel at the transmitter according to the present technology, therefore,the modified OFDM signal, e.g., a cyclically shifted representation ofthe OFDM signal, is analyzed to ensure that the pilot signals containedin the stream of OFDM symbols may be properly detected, and thereforewould likely be properly detected by a real receiver through a realcommunication channel.

According to another embodiment, the model is implemented using at leastone lookup table, based on previous applied time shifts (cyclic shifts).Assuming that the receiver has a specified margin for time shiftsbetween successive symbols or data blocks, the lookup table can thenestimate the tolerable range of delay to be added or subtracted from thesuccessive symbol or block, that will still be within the operatingrange of the receiver. According to this model, a demodulation is notper se required. The lookup table may in some cases be predetermined,but in others it can be adaptive. For example, different receivers mayimplement the standard differently, and thus have different tolerancefor variations in delay. Since the identification of the receiver maynot be available for the transmitter, it may be convenient to test therange of permissible delays at the beginning of a communicationssession, using the occurrence of retransmission requests to indicate therange of abilities. Note also that packets from the receiver to thetransmitter, such as retransmission requests, may be analyzed forcertain attributes of the channel conditions, such as relative speed(Doppler shift).

Another option is to modify the OFDM signal during all or a portion ofthe period in a manner which deviates from a standard protocol, whichis, for example an IEEE-802 OFDM standard, WiFi, WiMax, DAB, DVB,non-orthogonal multi access schemes, 3G, 4G, or 5G cellularcommunication, LTE or LTE-Advanced signals, or the like, but which doesnot substantively increase a predicted BER (bit error rate) of astandard or specific receiver. For example, if the PAPR is high for asmall portion a symbol period, such that if during a portion of thesymbol period, one or more subcarriers were eliminated or modified, thePAPR would be acceptable, and the signal at the receiver would havesufficient information to be decoded using a standard receiver withoutsignificant increase in BER, then the transmitter could implement suchmodifications without need to transmit side information identifying themodifications which are necessary for demodulation. Another possibledeviation is, for example, to frequency shift the signal (which mildlyviolates the orthogonality criterion), within the tolerance of areceiver to operate within a range of Doppler shifts which areequivalent to frequency shifts.

Consider two OFDM signals that are being combined as in FIG. 10. Forsimplicity, call Signal 1 (S1) the reference signal, and Signal 2 (S2)the modified signal. During each OFDM symbol period, the basebanddigital data bits for each signal will be stored in memory. Assume thatthe Preamble has been stripped off, but the Cyclic Prefix CP remains. Asindicated in FIG. 10 for one embodiment of the invention, the bits forthe reference signal S1 are stored in a first-in-first-out (FIFO) shiftregister (SR). The corresponding bits for the modified signal S2 arestored in a circular shift register (CSR), so configured that the datacontained can be rotated under program control. The data for bothsignals are first up-converted to an intermediate frequency (IF) andthen combined (added), while maintaining digital format at a samplingfrequency increased over the digital data rate. The combined IF signalsare then subjected to a PAPR test, to determine whether the peak powerlevel is acceptable, or, in other embodiments, whether other criteriaare met. This might correspond, for example, to a PAPR of 9 dB. If thetest is passed, then the data bits for the combined OFDM symbols areread out, to be subsequently reassembled into the full OFDM frame andup-converted to the full RF, for further amplification in the PA andtransmission. According to another embodiment, a combined OFDMrepresentation of the combined data is itself the source for theup-conversion.

More generally, once the parametric transformation (relative time-shift)to achieve the desired criteria is determined, the final signal is thenformulated dependent on that parameter or a resulting representation,which may be the digital data bits of the baseband signal or a convertedform thereof; in the latter case, the system may implement a series oftransformations on the data, some of which are redundant or failed,seeking an acceptable one or optimum one; once that is found, it may notbe necessary to repeat the series of transformations again. Likewise,the option of reverting to the original digital data and repeating thedetermined series of transformations allows a somewhat differentrepresentation to be formed in the register, for example one which issimplified or predistorted to allow consideration of analog componentperformance issues in the combining test.

Even more generally, the technique provides that each signal to becombined is provided with a range of one or more acceptable parameters,which may vary incrementally, algorithmically, randomly, or otherwise,and at least a portion of the possible combinations tested and/oranalyzed for conformity with one or more criteria, and thereafter thecombination of OFDM signals implemented using the selected parameter(s)from a larger set of available parameters. This parametric variation andtesting may be performed with high speed digital circuits, such assuperconducting logic, in a serial fashion, or slower logic withparallelization as necessary, though other technologies may be employedas appropriate and/or necessary, including but not limited to opticalcomputers, programmable logic arrays, massively parallel computers(e.g., graphic processors, such as nVidia Tesla® GPU, ATI Radeon R66,R700), and the like. The use of superconducting digital circuits may beadvantageous, for example, where a large number of complex computationswhich make significant use of a specialized high speed processor, suchas where a large number of independent receivers are modeled as part ofa transmitter optimization.

In the preferred embodiment, at any state of the tests over theparametric range, if the test is not passed, a control signal is fedback to the register, e.g., CSR, which rotates the data bits of themodified signal S2. The shifted data is then combined with the initialstored data from S1 as before, and the PAPR re-tested. This is repeateduntil the PAPR test is passed. A similar sequence of steps isillustrated in FIG. 10, where stripping off the preamble and reattachingit at the end are explicitly shown. It is noted that, in some cases, thetests may be applied in parallel, and therefore a strictly iterativetest is not required. This, in turn, permits use of lower speed testinglogic, albeit of higher complexity. Likewise, at each relativetime-shift, a secondary parameter may also be considered.

For example, a secondary consideration for optimal combining may bein-band (non-filterable) intermodulation distortion. Thus, at each basicparametric variation, the predicted in-band intermodulation distortion,expressed, for example, as a power and/or inferred BER, may becalculated. This consideration may be merged with the PAPR, for example,by imposing a threshold or optimizing a simple linear combination “costfunction”, within an acceptable PAPR range.

While there may be some delays in this Shift-and-Add process (SAA), thetime for the entire decision algorithm, including all iterations, mustnot exceed the expanded symbol time T+ΔT. We have described a serialdecision process in FIGS. 4 and 10. As discussed above, in some cases,it may be preferable to carry out parts of this process in parallel,using multiple CSRs with different shifts and multiple parallel PAPRtests, in order to complete the process more quickly. This isillustrated in FIG. 11, which suggests parallel memories (shown here asRAMs), each with an appropriate time shift, where the minimum PAPR isselected to send to the RF subsystem. The optimum tradeoff betweencircuit speed and complexity will determine the preferred configuration.

In some situations, the search for an optimum combined signal requiresvast computational resources. In fact, heuristics may be available tolimit the search while still achieving an acceptable result. In the caseof a PAPR optimization, generally the goal is to test for limited, lowprobability “worst case” combinations of symbols. If the raw digitaldata is available, a lookup table may be employed to test for badcombinations, which can then be addressed according to a predeterminedmodification. However, for multi-way combinations of complex symbolsthis lookup table may be infeasible. On the other hand, the individualOFDM waveforms may each be searched for peaks, for example 6 dB abovemean, and only these portions of the signal analyzed to determinewhether there is a temporal alignment with the peaks of other OFDMsignals; if the peaks are not temporally synchronized, then apresumption is made that an unacceptable peak will not result in thefinal combined signal. This method makes a presumption that should bestatistically acceptable, that is, that only portions of an OFDMwaveform that are themselves relative peaks will contribute to largepeaks in the combination of OFDM signals. This method avoids serialtesting of sequential parametric variations, and rather simply avoidsworst case superpositions of a binary threshold condition.

Although these figures focus on the case of reducing PAPR for thecombination of two OFDM channels, this method is not limited to twochannels. Three or more channels can be optimized by a similar method ofcircular time shifts, followed by PAPR tests. Furthermore, althoughcyclic shifting is presented as a preferred embodiment of the proposedmethod, this is intended to represent a specific example of a moregeneral signal transformation. Any such transformation that encodes thesame information, and can be decoded (without error) by the receiverwithout the transmission of additional side information, would serve thesame purpose. The identification of such transformations depends on thedetails of present and future protocols for wireless signalcommunication systems.

Finally, both the codebook LUT and the signal transformation mayincorporate other digital methods to improve signal fidelity, such aspredistortion (to compensate for power amplifier nonlinearity) andmulti-antenna transmission (MIMO). In this way, the carrier aggregationmethod of the present invention can accommodate new approaches toincrease data rate and decrease noise.

It is therefore an object to provide a method for controlling a combinedwaveform, representing a combination of at least one multiple subchannelmultiplexed (MSM) signal with another signal, comprising: receivinginformation to be communicated from a transmitter to a receiver throughthe at least one MSM signal according to a predetermined protocol, theMSM signal comprising pilot signals, within at least one subchannel forat least a portion of time, having predefined characteristicssufficiently independent of the information to be communicated, topermit receiver prediction of a communication channel state with respectto varying communication channel conditions; storing a model of thereceiver with respect to prior communications and a combination of theMSM signal with the other signal in a memory, the model being forpredicting a receiver ability to demodulate the information and areceiver ability to predict the channel state, over a range of at leastone parameter representing an alteration in the state of thecombination; and defining, with an automated processor, the combinationof the MSM signal with the other signal which is predicted to permitsufficient receiver estimation of the channel state to demodulate theinformation and which further meets at least one fitness criteriondistinct from the receiver estimation of the channel state and beingdependent on both MSM signal and the other signal, with respect to atleast two different values for the at least one variable parameter.

It is also an object to provide a system for controlling a combinedwaveform, representing a combination of at least one multiple subchannelmultiplexed (MSM) signal according to a predetermined protocol to becommunicated from a transmitter to a receiver with another signal, theMSM signal comprising pilot signals, within at least one subchannel forat least a portion of time, having predefined characteristicssufficiently independent of the information to be communicated, topermit receiver prediction of a communication channel state with respectto varying communication channel conditions, comprising: a model of areceiver stored in a memory, dependent on prior communications and acombination of the MSM signal with the other signal in a memory, forpredicting a receiver ability to demodulate information and a receiverability to predict the channel state, over a range of at least oneparameter representing an alteration in the state of the combination;and an automated processor configured to define the combination of theMSM signal with the other signal which is predicted to permit sufficientreceiver estimation of the channel state to demodulate the informationand which further meets at least one fitness criterion distinct from thereceiver estimation of the channel state and being dependent on both MSMsignal and the other signal, with respect to at least two differentvalues for the at least one variable parameter.

It is a further object to provide a method for controlling a combinedwaveform, representing a combination of signals, the signals comprisingat least one multiple subchannel multiplexed signal having informationmodulated in respective subchannels, with a modulated signal,comprising: receiving information to be communicated from a transmitterto a receiver through the at least one multiple subchannel multiplexedsignal according to a predetermined protocol, the multiple subchannelmultiplexed signal comprising pilot signals, within at least onesubchannel for at least a portion of time, having predefinedcharacteristics sufficiently independent of the information to becommunicated, to permit receiver prediction of a communication channelstate with respect to varying communication channel conditions; storinga model of the receiver with respect to a combination of the multiplesubchannel multiplexed signal with the modulated signal in a memory, themodel being for predicting a receiver ability to demodulate theinformation and a receiver ability to predict the channel state, over arange of at least one parameter representing available alterations inthe state of the combination; and defining, with an automated processor,the combination signals which is predicted to permit sufficient receiverestimation of the channel state to demodulate the information fromrespective subchannels and which further meets at least one fitnesscriterion distinct from the receiver estimation of the channel state andbeing dependent on both the multiple subchannel multiplexed signal andthe modulated signal, with respect to at least two different values forthe at least one parameter.

It is another object to provide a system for controlling a combinedwaveform, representing a combination of signals, the signals comprisingat least one multiple subchannel multiplexed signal having informationmodulated in respective subchannels, with a modulated signal,comprising: an input configured to receive information to becommunicated from a transmitter to a receiver through the at least onemultiple subchannel multiplexed signal according to a predeterminedprotocol, the multiple subchannel multiplexed signal comprising pilotsignals, within at least one subchannel for at least a portion of time,having predefined characteristics sufficiently independent of theinformation to be communicated, to permit receiver prediction of acommunication channel state with respect to varying communicationchannel conditions; a memory configured to store a model of the receiverwith respect to an ability to estimate a channel state demodulate theinformation, from a combination of the multiple subchannel multiplexedsignal with the modulated signal; at least one automated processorconfigured to: define a plurality of alternate representations ofdiffering combinations of the multiple subchannel multiplexed signalwith the modulated signal, differing with respect to at least oneparameter, wherein the at least one parameter has a range which includesat least one value that impairs an ability to estimate a channel stateby the receiver; and select at least one combination of the multiplesubchannel multiplexed signal with the modulated signal which ispredicted based on the model to permit sufficient receiver estimation ofthe channel state and to demodulate the information from respectivesubchannels with respect to the defined plurality of alternaterepresentations of differing combinations.

It is still another object to provide a system for controlling acombined waveform, representing a combination of at least one multiplesubchannel multiplexed signal according to a predetermined protocol tobe communicated from a transmitter to a receiver with a modulatedsignal, the multiple subchannel multiplexed signal comprising pilotsignals, within at least one subchannel for at least a portion of time,having predefined characteristics sufficiently independent ofinformation to be communicated, to permit receiver prediction of acommunication channel state with respect to varying communicationchannel conditions, comprising: a model of a receiver stored in amemory, dependent on a combination of the multiple subchannelmultiplexed signal with the modulated signal in a memory, for predictinga receiver ability to demodulate information and a receiver ability topredict the channel state, over a range of at least one parameterrepresenting an alteration in the state of the combination; and anautomated processor configured to define the combination of the multiplesubchannel multiplexed signal with the modulated signal which ispredicted to permit sufficient receiver estimation of the channel stateto demodulate the information and which further meets at least onefitness criterion distinct from the receiver estimation of the channelstate and being dependent on both the multiple subchannel multiplexedsignal and the modulated signal, with respect to at least two differentvalues for the at least one parameter.

Another object provides an apparatus for combining a plurality ofsignals in a respective plurality of channels, each signal comprising aset of concurrent phase and/or amplitude modulated components within achannel, comprising: a processor configured to: receive informationdefining each of the plurality of signals; transform a representation ofat least one signal in a plurality of different ways, each transformedrepresentation representing the same information, analyze respectivecombinations of each transformed representation with informationdefining at least one other signal, with respect to at least twodifferent fitness criteria, and select at least one respectivecombination as being fit according to the analysis with respect to theat least two different fitness criteria; and an output port configuredto present at least one of an identification of the selected at leastone respective combination, the selected at least one respectivecombination, and information defining the selected at least onerespective combination, wherein at least one of the criteria relates toa predicted ability of a receiver to estimate a channel state of acommunication channel, and wherein at least one transformedrepresentation impairs an ability of the receiver to successfullyestimate the channel state of the communication channel.

It is a still further object to provide an apparatus for combining aplurality of signals in a respective plurality of channels, each signalcomprising a set of phase and/or amplitude modulated orthogonalfrequency components within a channel, comprising: a processorconfigured to receive information defining each of the plurality ofsignals, being represented as a plurality of orthogonal frequencymultiplexed signal components, transform a representation of at leastone signal in at least two different ways, each transformedrepresentation representing the same information, analyze with respectto at least two different fitness criteria a plurality of differentcombinations of the plurality of signals, each of the plurality ofrepresentations including alternate representations of at least onesignal subject to at least two different transformations, and select acombination based on the analyzing which meets each of the at least twocriteria; and an output port configured to present at least one of anidentification of the selected combination, the selected combination,and information defining the selected combination, wherein at least oneof the criteria relates to a predicted ability of a receiver to estimatea channel state of a communication channel based on pilot sequenceswithin the representation, wherein at least one transformation of therepresentation impedes an ability of the receiver to successfullyestimate the channel state for demodulating the information.

It is also an object to provide an apparatus for controlling a combinedwaveform, representing a combination of at least two signals, eachhaving a plurality of signal components and conveying information,comprising: an input port configured to receive information defining theat least two signals; an automated processor configured to: transform afirst of the signals into at least two representations of the conveyedinformation within a range of transformations, along with pilot signalinformation which varies in frequency and which is selectivelycommunicated over time, to permit a receiver to estimate a channelstate, having prohibited combinations of transformation parameters andpilot signal information, combine a transformed representation of afirst of the signals with a second of the signals, to define at leasttwo alternate combinations representing the conveyed information, andselect one combination which meets a predetermined criterion and whichpermits the receiver to at least estimate the channel state; and anoutput port configured to output information representing a respectivecombined waveform comprising the selected one combination of thetransformed representation which meets the predetermined criterion.

A plurality of sets of combined waveforms may be formed, each combinedwaveform representing a respective combination of the multiplesubchannel multiplexed signal with the modulated signal at a respectivevalue of the at least one parameter, and said defining comprisesselecting a respective defined waveform which meets the at least onefitness criterion. The modulated signal is typically modulated withsecond information (which may be intelligence or a pseudorandom noisesequence), and the automated processor may further define thecombination of the multiple subchannel multiplexed signal with themodulated signal in a manner which is predicted to permit demodulationof the second information from the modulated signal.

The MSM signal may be, for example, an orthogonal frequency multiplexedsignal, but is not so limited. In particular, the subcarriers need notbe orthogonal, and indeed, the subcarriers need not be distributedaccording to frequency, though such an arrangement is presentlypreferred, especially to the extent that the receivers are standard OFDMreceivers. The MSM signal may also be a wavelet encoded signal, in whichcase the discrete wavelet transform (DWT) and corresponding inversewavelet transform (IWT) generally replace the FFT and IFT employedwithin OFDM communications. An orthogonality constraint may be relaxedsuch that the receiver in the estimated state can demodulate theinformation, without strictly meeting the constraint.

In general, MSM signals are intended to be communicated to, and receivedby, mobile receivers, and therefore the communication protocol providestolerance to various types of interference and distortion. For example,time varying multipath, Doppler shifts, and the like, are tolerable. Thepresent technology can model the receiver with respect to the tolerance,for example by calculating a bit error rate, or data throughput rate(dependent on retransmission of packets and error detection andcorrection (EDC) code burden), and optimizing the combined signal.

The plurality of signals may each comprise orthogonal frequency divisionmultiplexed signals. A first combination and a second combination basedon different values of the variable parameter may differ with respect toat least one of (a) a relative timing of a modulation of the frequencycomponents of a first signal with respect to a second signal and (b) arelative phase of the frequency components of the first signal withrespect to the second signal. The at least one criterion may comprise apeak to average power ratio (PAPR). The selected at least one respectivecombination may comprise a combination that is beneath a threshold peakto average power ratio.

The at least one fitness criterion may comprise a peak to average powerratio (PAPR).

The signals may comprise orthogonal frequency division multiplexed(OFDM) signals. At least one signal may be an orthogonal frequencydivision multiplexed stream which is compatible with at least oneprotocol selected from the group of an IEEE 802.11 protocol, an IEEE802.16 protocol, a 3GPP-LTE downlink protocol, LTE-Advanced protocol, aDAB protocol and a DVB protocol, wherein a receiver compliant with theat least one protocol can demodulate the at least two respectivelydifferent combinations of without requiring additional information to betransmitted outside of the protocol.

The at least two different values of the variable parameter maycorrespond to signals that differ respectively in a cyclic time shift ina modulation sequence.

The at least two different values of the variable parameter maycorrespond to signals that differ respectively in a cyclic time shift ina modulation sequence, and the at least one fitness criterion maycomprises a peak to average power ratio (PAPR). An alternaterepresentation which results in a lowest peak to average power ratio, ora peak to average power ratio below a threshold, may be selected forcombination.

At least one of an intermediate frequency at a frequency below about 125MHz and a radio frequency representation at a frequency greater thanabout 500 MHz of the defined combination of signals may be predistorted.

The automated processor may comprise superconducting digital logiccircuits. Alternately, the automated processor may comprise aprogrammable logic device, programmable logic array, CPLD, RISCprocessor, CISC processor, SIMD processor, general purpose graphicsprocessing unit (GPGPU) or the like, implemented in silicon technology,superconducting digital logic circuits, or other types of logic.

Meeting the fitness criterion may comprise analyzing with respect todynamic range of a respective combination, or analyzing with respect toa predicted error rate of a reference receiver design for one of thesignals, or analyzing with respect to a peak to average power ratio ofthe combined waveform and a predicted error rate of a receiver for oneof the signals, or analyzing a clipping distortion of the combinedwaveform, for example.

The combined waveform may be a digital representation that is sampled ata data rate higher than the corresponding data rates of any of thecomponent signals or a digital representation of an intermediatefrequency representation of the combined signal, for example.

Generating the combined waveform may comprise outputting a set ofparameters for converting a digital baseband signal into the selectedcombined signal.

The method may further comprise predistorting at least one of anintermediate frequency and a radio frequency representation of theselected combined signal. The predistorting may compensate for at leasta portion of one or more of an analog non-linearity, a transmissionchannel impairment, and a receiver characteristic of an analog radiocommunication system communicating using the selected combined signal.The predistorting may also compensate for a non-linear distortion of apower amplifier which amplifies the selected combined signal.

Each of the at least two signals may comprise an orthogonal frequencydomain multiplexed signal having a cyclic prefix, and wherein the twoalternate representations differ in a respective cyclic time shift.

Each of the at least two signals may be received as an orthogonalfrequency division multiplexed signal conforming to a communicationsprotocol, at least one of the signals may be modified to generate the atleast two alternate representations, and the at least one fitnesscriterion may comprise a peak to average power ratio of the combinedsignal, wherein the selected combined signal is a combined signalrepresenting a lowest peak to average power ratio or a peak to averagepower ratio below a predetermined threshold.

The receiver model may incorporate prior pilot signals in definingacceptable values of the variable parameter that are selected forfurther evaluation according to the fitness criterion.

The selection of acceptable values of the variable parameter may beimplemented with the use of an adaptive lookup table memory. Theautomated processor may be configured to retrieve values from the lookuptable for the selection of the at least one combination.

The receiver model may extrapolate prior pilot signals to generate areference signal during time periods when a current pilot signal is notavailable.

A buffer memory may be used to store the input signals until thepreferred combination for transmission is defined. The buffer memory maycomprise at least one shift register.

The evaluation of the fitness criterion for the combination may beimplemented in parallel for the different values of the variableparameter.

The automated processor may comprise a programmable gate array.

Each of the at least two signals may be received as an orthogonalfrequency division multiplexed signal conforming to a communicationsprotocol, at least one of the signals may be modified to generate the atleast two alternate representations, each of which can be demodulated bya receiver compatible with the protocol without requiring receipt ofadditional information outside of the communications protocol, and theat least one criterion may comprise a peak to average power ratio of thecombined signal. The at least one criterion may comprise a peak toaverage power ratio of the combined signal. The selected combined signalmay be a combined signal representing a lowest peak to average powerratio, or a peak to average power ratio within a threshold level.

The MSM signal may be an orthogonal frequency multiplexed (OFDM) signalhaving a plurality of subcarriers at different frequencies whichconcurrently communicate the information. The MSM signal and the othersignal may each be within a different communication channel and beprocessed as a combined analog signal in at least one analog signalprocessing component having a non-linear distortion. The MSM signal maycomply with a predetermined protocol which selectively inserts the pilotsignals in a plurality of the subcarriers at different times and atdifferent frequencies, to estimate the channel state. The at least oneparameter may comprise a cyclic shift of digital data representing theMSM signal, wherein the model predicts an ability of a receiver whichcomplies with an OFDM protocol to detect the pilot and estimate thechannel state subject to at least two different cyclic shifts. The atleast one fitness criterion may be dependent on the non-lineardistortion of the combination of the MSM signal with the other signal inthe analog signal processing component.

It is a further object to provide an apparatus for controlling acombined waveform, representing a combination of at least two signalshaving a plurality of signal components, comprising: an input portconfigured to receive information defining the at least two signals; aprocessor configured to: transform the information defining each signalinto a representation having a plurality of components, such that atleast one signal has alternate representations of the same informationalong with further information to permit a receiver to estimate achannel state, and combining the transformed information using thealternate representations, to define at least one combination whichmeets a predetermined criterion and which permits the receiver toestimate the channel state, wherein the transform is adapted to defineat least one alternate representation which fails to permit the receiverto estimate the channel state; and an output port configured to outputinformation representing a respective combined waveform comprising thecombination of the transformed information from each of the at least twosignals which meets the criterion.

Thus, the transmitter may combine signals in such a way that thecombination may violate a first criterion, but that the same informationmay be combined by altering the combination without violating the firstcriterion. However, the altered combination may violate a secondcriterion, that the un-altered combination generally does not violate.The processor seeks to find an alteration, which may require a searchthrough a range, which meets both the first criterion and the secondcriterion. The second criterion relates to a communication from thetransmitter to the receiver of pilot information which calibrates thereceiver and for example permits the receiver to estimate the channelstate. The pilot signals may be sparsely inserted into the combinedsignal, and the receiver may estimate the channel state based on aseries of communicated pilot signals over a series of data frames.

A first combination and a second combination of the transformedinformation may differ with respect to at least one of (a) a relativetiming of a modulation of the frequency components of a first signalwith respect to a second signal and (b) a relative phase of thefrequency components of a signal, and the at least one criterion maycomprise a peak to average power ratio (PAPR). At least one signal maybe an orthogonal frequency division multiplexed stream which iscompatible with at least one protocol selected from the group of an IEEE802.11 protocol, an IEEE 802.16 protocol, a 3GPP-LTE downlink protocol,a DAB protocol and a DVB protocol, wherein a receiver compliant with theat least one protocol can demodulate the at least two respectivelydifferent combinations without requiring additional information to betransmitted outside of the protocol. Each transformed representation maydiffer respectively in a cyclic time shift in a modulation sequence. Theat least two criteria comprise a peak to average power ratio (PAPR),wherein an alternate representation which results in a peak to averagepower ratio within a threshold maximum peak to average power ratio isselected for combination.

The at least two alternate representations may differ respectively in acyclic time shift in a modulation sequence, and the at least onecriterion may comprise a peak to average power ratio (PAPR), wherein analternate representation which results in a lowest peak to average powerratio is selected for combination.

The processor may comprise at least one of superconducting digitalcircuit logic and a complex programmable logic device (CPLD).

The output port may be configured to output the selected combined signalas a direct conversion from a digital representation of the combinedsignal to a radio frequency analog signal adapted for transmissionwithout frequency modification.

The processor may be further configured to predistort at least one of anintermediate frequency and a radio frequency representation of theselected combined signal.

The predistortion may compensate for at least a portion of one or moreof an analog non-linearity, a transmission channel impairment, and areceiver characteristic of an analog radio communication systemcommunicating using the selected combined signal.

The signals may comprise orthogonal frequency division multiplexedsignals. A first combination and a second combination of the transformedinformation may differ with respect to at least one of (a) a relativetiming of a modulation of the frequency components of a first signalwith respect to a second signal and (b) a relative phase of thefrequency components of a signal. The at least one criterion maycomprise a peak to average power ratio (PAPR). The selected combinationmay comprise a combination with a lowest peak to average power ratio.

Each transformed representation may differ respectively in a cyclic timeshift in a modulation sequence, and wherein the orthogonal frequencydivision multiplexed signals are compatible with at least one protocolselected from the group of an IEEE 802.11 protocol, an IEEE 802.16protocol, a 3GPP-LTE downlink protocol, a DAB protocol and a DVBprotocol, wherein a receiver compliant with the at least one protocolcan demodulate the at least two respectively different combinations ofwithout requiring additional information to be transmitted outside ofthe protocol, each transformed representation differing respectively ina cyclic time shift in a modulation sequence, and the at least onecriterion may comprise a peak to average power ratio (PAPR), wherein analternate representation which results in a lowest peak to average powerratio is selected for combination. The processor may comprise at leastone of superconducting digital circuit logic and a complex programmablelogic device (CPLD). The processor may analyze a nonlinear distortion ofthe combined waveform in a model of an amplifier, and further predistortat least of at least one component of the selected combination and theselected combination.

Further objects will become apparent through a review of the detaileddescription and figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show typical behavior of an orthogonal frequency-domainmultiplexed channel in the frequency and time domains, respectively.

FIG. 2 represents a time-frequency resource grid for an OFDM channel,showing typical locations of pilot symbols according to the protocol forLTE.

FIG. 3 shows the combination of two OFDM channels in a transmitter usinga double-upconversion method.

FIG. 4 provides a simple block diagram showing how two OFDM channels maybe combined, wherein the data bits of one OFDM channel may be cyclicallyshifted in order to reduce the peak-to-average power ratio (PAPR).

FIG. 5 shows a block diagram of an OFDM communication system thatincorporates the shift-and-add algorithm in the transmitter and a pilotphase tracker in the receiver.

FIG. 6 shows a block diagram of an OFDM communication system thatenables the SAA algorithm for a resource grid as in FIG. 2.

FIG. 7 shows a top-level flowchart for a generalized carrier aggregationmethod of the present invention.

FIG. 8 shows a block diagram of an OFDM communication system, wherebythe receiver generates an equalized resource grid based on an array ofpilot symbols as in FIG. 2.

FIG. 9 shows a block diagram that represents the process of equalizingthe resource grid at the receiver using an array of pilot symbols as inFIG. 2.

FIG. 10 shows the structure of two OFDM channels, with cyclic shiftingof the data for one channel in order to reduce the PAPR of the combinedsignal.

FIG. 11 provides a block diagram showing memory storage of multipleshifted replicas of data from an OFDM channel, with selection of onereplica corresponding to minimizing the PAPR of the combined signal.

FIG. 12A shows a typical 64QAM constellation diagram for a simulatedOFDM received signal without added noise.

FIG. 12B shows a 64QAM constellation diagram for a simulated OFDMreceived signal with noise added.

FIG. 13A shows a probability plot for PAPR of the carrier aggregation ofsimulated OFDM signals, showing reduced PAPR for the method of theinvention.

FIG. 13B shows a probability plot for PAPR of the carrier aggregation ofsimulated OFDM signals, including the effect of digital predistortion.

FIG. 13C shows a block diagram of the simulation with results shown inFIGS. 13A and 13B.

FIG. 14 shows a block diagram of a system according to one embodiment ofthe invention.

FIG. 15 shows a block diagram of a system according to one embodiment ofthe invention, including digital predistortion to compensate for anonlinear analog amplifier.

FIG. 16 shows a block diagram of a system according to the invention,implemented on an FPGA.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

OFDM channels are comprised of many sub-channels, each of which is anarrow-band signal (FIG. 1A). An OFDM channel itself has a time-varyingenvelope, and may exhibit a substantial PAPR, typically 9-10 dB.However, if two separate similar OFDM channels are combined, theresulting signal will typically exhibit PAPR of 12-13 dB, for a gain of3 dB. This is unacceptably large, since it would require a poweramplifier with 4 times the capacity to transmit a combined signal thataverages only 2 times larger.

A preferred embodiment therefore provides a PAPR reduction method whichreduces the PAPR of a two OFDM channel combined signal from 12-13 dBback down to the 9-10 dB of the original components. This ˜3 dBreduction in PAPR is preferably accomplished without degradation of thesignal, and without the need to transmit any special side informationthat the receiver would need to recover the OFDM symbols. Further, thealgorithm is simple enough that it can be implemented in any hardwaretechnology, as long as it is sufficiently fast.

Conventional methods of PAPR reduction focus on combining thesub-channels and generating a single OFDM channel without excessivePAPR. The present technique can be viewed in certain respects as acombination of Partial Transmit Sequence (PTM) and Selected Mapping(SLM).

In traditional PTS, an input data block of N symbols is partitioned intodisjoint sub-blocks. The sub-carriers in each sub-block are weighted bya phase factor for that sub-block. The phase factors are selected suchthat the PAPR of the combined signal is minimized.

In the SLM technique, the transmitter generates a set of sufficientlydifferent candidate data blocks, all representing the same informationas the original data block, and selects the most favorable fortransmission (lowest PAPR without signal degradation).

The present hybrid approach combines elements of PTS and SLM for summedcarrier modulated signals. The various cyclic time-shifts of theoversampled OFDM waveform are searched, and the time-shift with thelowest PAPR selected. One OFDM signal is used as reference and the othercarrier modulated signal(s) are used to generate the time-shifts, in amanner similar to PTS. The search window is determined by the cyclicprefix length and the oversampling rate.

While the phase space of possible combinations of shifts increasestremendously, it would not be necessary to explore all suchcombinations. In general, very high values of the PAPR occur relativelyrarely, so that most time shifts starting with a high-PAPR state wouldtend to result in a reduction in PAPR. Shifts in multiple channels couldbe implemented sequentially or in parallel, or in some combination ofthe two. Thus, for example, any combination with a PAPR within anacceptable range is acceptable, any unacceptable PAPR states occur 1% ofthe time, the search space to find an acceptable PAPR would generally be<2% of the possible states. On the other hand, if other acceptabilitycriteria are employed, a larger search space may be necessary orappropriate. For example, assuming that there is a higher cost fortransmitting a higher PAPR signal, e.g., a power cost or an interferencecost, then a formal optimization may be appropriate. Assuming that noheuristic is available for predicting an optimal state, a full search ofthe parametric space may then be appropriate to minimize the cost.

This differs from conventional approaches, wherein different OFDMchannels are independent of one another, with separate transmit chainsand without mutual synchronization. Further, the conventional approachesoperate directly on the baseband signals. In contrast, the presentmethod evaluates PAPR on an up-converted, combined signal thatincorporates two or more OFDM channels, and the symbol periods for eachof these channels must be synchronized.

This will not cause problems at the receivers, where each channel isreceived and clocked independently.

Some conventional approaches to PAPR are based on clipping, but theseinevitably produce distortion and out-of-band generation. Some otherapproaches avoid distortion, but require special transformations thatmust be decoded at the receive end. These either require sendingside-information, or involve deviations from the standard OFDMcommunication protocols. The present preferred approach has neithershortcoming.

OFDM channels used in cellular communications, may be up to 10 or 20 MHzin bandwidth. However, these channels might be located in a much broaderfrequency band, such as 2.5-2.7 GHz. So one might have a combination oftwo or more OFDM channels, each 10 MHz wide, separated by 100 MHz ormore. A 10 MHz digital baseband signal may be sampled at a rate as lowas 20 MS/s, but a combined digital signal covering 100 MHz must besampled at a rate of at least 200 MS/s.

In a preferred embodiment, the signal combination (including theup-conversion in FIG. 3) is carried out in the digital domain at such anenhanced sampling rate. The PAPR threshold test and CSR control are alsoimplemented at the higher rate. This rate should be fast enough so thatmultiple iterations can be carried out within a single symbol time(several microseconds).

In order to verify the expectation that the circular time-shift permitsreduction in PAPR for combined OFDM channels, without degrading systemperformance, a full Monte-Carlo simulation of OFDM transmission andreception was carried out. The block diagram of this simulation issummarized in FIG. 6, which represents the “Carrier AggregationEvaluation Test Bench”, and shows a transmitter that combines OFDMsignals at frequencies F₁ and F₂, subject to the SAA algorithm for PAPRreduction. At the receive end, this is down-converted and the signal atF₂ is recovered using a standard OFDM receiver. Along the way,appropriate Additive White Gaussian Noise (AWGN) is added to thechannel. The parameters for the Carrier

Aggregation simulations include the following. Each packet contains 800bytes of information, which is modulated over several OFDM symbolperiods, and the modulation is 64-QAM (64-quadrature amplitudemodulation). Each SNR point is run until 250 packet errors occur. Thecyclic prefix is set to ⅛ of the total symbol time. Carriers atfrequencies F₁ and F₂ are spaced sufficiently that their spectra do notoverlap. The oversampling rate is a factor of 8. Finally, a raisedcosine filter was used, with a very sharp rolloff, with a samplingfrequency F_(s)=160 MHz, and a frequency cutoff F_(c)=24 MHz. FIG. 12Ashows an example of a constellation chart of the 64-QAM received signalsfor the simulation without noise, where a time shift has been appliedthat is expected to be compatible with the interpolation equalizer ofthe receiver. In this example, no pilot symbol was transmitted duringthis time period. The clustering indicates that each bit is receivedwithin its required window, with no evidence of bit errors. Moregenerally, no degradation of the signal was observed for an allowabletime shift, as expected. FIG. 12B shows a similar 64-QAM constellationchart for the simulation with added Gaussian noise typical of apractical wireless communication system. Again, the simulation showsproper reception of the signal with no significant increase in biterrors.

FIG. 13A shows the simulated PAPR distribution for a combination of twoOFDM signals, combined according to an embodiment of the invention. TheComplementary Cumulative Distribution Function (CCDF) represents theprobability that the signal has a PAPR greater than a given value. Forpractical purposes, a CCDF of 10⁻⁴ can be used to define the effectivePAPR of a particular waveform. Each of the two component signals has aPAPR of 11 dB (top curve). The combination of the two signals withoutmodification would lead to an increase in PAPR of almost 3 dB (notshown). In contrast, combination using the Codebook Pre-Weightingalgorithm of the present invention leads to a decrease of almost 2 dB toabout 9 dB (bottom curve). This benefit would be reduced if thisCodebook approach is not applied (middle curve).

FIG. 13B shows the effect of applying digital predistortion (DPD) inaddition to Crest Factor Reduction (CFR), as indicated in the simulationblock diagram of FIG. 13C. FIG. 13C shows the combination of three OFDMsignals, each corresponding to LTE signals of 20 MHz bandwidth. Theindividual baseband signals are sampled at 30.72 MHz, followed byupsampling to 122.8 MHz, offsetting the frequencies (using a digitalmultiplier), and adding together to form an IF signal with a 60 MHz bandcomprising three 20 MHz bands. This is then subject to Crest FactorReduction (CFR) according to the Codebook Weighting algorithm of thepresent invention, followed by upsampling (by a factor of two) anddigital predistortion (DPD, to simulate the saturation effect of anonlinear power amplifier PA). Finally, the predistorted signal is sentto a digital-to-analog converter (DAC) and then amplified in the PA. Thecurve in FIG. 13B labeled CFR Input shows the combined signal, while CFROutput shows the result of PAPR reduction. The curve labeled “+n SCAProcessing” (PlusN Smart Carrier Aggregation) corresponds to the signalas broadcast, including the effects of predistortion.

These simulations have confirmed not only that the SAA algorithm permitsreduction of PAPR in combined OFDM channels by ˜3 dB, but also that thisreduction is achieved without signal degradation and without the need tosend any special side information on the transformations in the transmitsignal. This can also be integrated with digital predistortion, withoutdegradation of the PAPR reduction.

A block diagram of a system according to one embodiment of the inventionis shown in FIG. 14, where at least one of the input signals isidentified as a multiple subchannel multiplexed (MSM) signal,essentially a generalization of an OFDM signal. The MSM signal isassumed to include pilot signals independent of the information content,which enable the signal to be properly received in the presence ofmulti-path, Doppler shift, and noise. Here the MSM signal and anothersignal are combined in a plurality of alternative aggregated signals,where each such alternative combination could be properly received forboth such signals at a receiver, without sending additionalside-information. A digital model of the receiver, which may incorporateprior transmitted signals, permits determination of which alternativecombinations correspond to MSM pilot signals that can be properlytracked at the receiver. Based on this criterion, and at least one othercriterion that may be associated with combined signal amplitude (such aspeak-to-average-power ratio or PAPR), one or more of the alternativecombinations is selected, which may be subject to further processing orselection, e.g., in an iterative selection process using variouscriteria, for transmission using an automated processor. This maypreferably be carried out using a digital IF signal, which is thenconverted to an analog signal in a digital-to-analog converter (DAC),and then upconverted to the full radio frequency signal in the standardway in an analog mixer before being amplified in the Power Amplifier andtransmitted via an antenna. Other types of RF modulators may also beemployed.

FIG. 15 represents a block diagram similar to that in FIG. 14, but withthe addition of digital predistortion modules that compensate fornonlinearities that may be present in nonlinear analog components suchas the Power Amplifier (including inherent non-linearities,signal-dependent delays, saturation and heating effects). Thepredistortion is preferably carried out on the alternative combinations,so that the selected combination(s) properly meet all criteria.

The predistortion may encompass correction of multiple distortionsources, and represent transformations of the signal in the time (delay)and/or frequency domains, amplitude and waveform adjustments, and may beadaptive, for example, to compensate for aging and environmentalconditions. In the case of multiple-input multiple-output (MIMO) radiotransmission systems (or other signal transmissions), the distortionmodel encompasses the entire signal transmission chain. This model mayinclude distinct models for the various multipaths, and therefore theselected alternative predistorted signal may represent an optimum forthe aggregate system, and not only the “principal” signal component.

One preferred implementation of the technique involves using a fastfield-programmable gate array (FPGA) with blocks for shift-registermemories, lookup tables, digital up-conversion, and threshold testing.This is illustrated in FIG. 16, which also shows the optional additionof digital predistortion. In this embodiment, the input digital basebandsignals (in the time domain) are first stored in memory registers withinthe FPGA, and the MSM signal S2 is transformed in a plurality of digitalprecoders. In one embodiment, these precoders may comprise circularshift registers (CSRs) with different values of the shift parameter. Inother embodiments, the range of parameter variation is not time (i.e.,the incremental variation in a CSR), but rather another parameter, suchas the time-frequency range of a wavelet transform. These shiftedversions are chosen so as to be compatible with pilot signal tracking inthe receiver, as determined by the lookup-table discriminator block.This LUT may take into account prior shifts, as shown in FIG. 16. FIG.16 shows several precoding schemas (e.g., circular shifts) beingprocessed in parallel, although serial processing is also possible. Eachbaseband signal to be combined is subjected to digital upconversion inthe digital upconverter DUC to a proper intermediate frequency (IF),with an increase in sampling rate as appropriate. Sample S1 and eachalternative S2 may then be digitally combined in the Digital IF Combinerunit. This is followed by optional digital predistortion in digitalpredistorters PD, before each alternative combination is sent to theThreshold Tester. The Threshold Tester may, for example, measure thePAPR of each alternative, and choose the alternative with the lowestPAPR.

Alternatively, an ultrafast digital technology, such asrapid-single-flux-quantum (RSFQ) superconducting circuits, may beemployed. As the number of OFDM channels being combined is increased,one needs either to increase the algorithm speed, or alternatively carryout a portion of the processing in parallel.

This method may also be applied to a reconfigurable system along thelines of cognitive radio, wherein the channels to be transmitted may bedynamically reassigned depending on user demand and available bandwidth.Both the number of transmitted channels and their frequency allocationmay be varied, under full software control. As long as all channelsfollow the same general symbol protocol and timing, one may apply asimilar set of Shift-and-Add algorithms to maintain an acceptable PAPRfor efficient transmission.

What is claimed is:
 1. A radio transmitter, comprising: at least oneinput port configured to receive information defining at least twoorthogonal frequency division multiplexed (OFDM) signals, each havingOFDM components within OFDM subchannels of a respective separate OFDMchannel, each respective OFDM signal being compliant with an OFDM signalcommunication protocol, such that an OFDM signal communication protocolcompliant receiver can demodulate the respective OFDM signal; at leastone automated processor, configured to: define at least two alternatecombinations of the at least two OFDM signals occupying the separateOFDM channels, each alternate combination having the OFDM componentswithin the respective OFDM subchannels of the respective separate OFDMchannels, each of the at least two OFDM signals within each of the atleast two alternate combinations of the at least two OFDM signals beingcompliant with the OFDM signal communication protocol, such that theOFDM signal communication protocol compliant receiver can demodulate therespective OFDM signals of each respective alternate combination withoutside information outside of the OFDM signal communication protocol; andselect one of the alternate combinations of the at least two OFDMsignals based on at least one selection criteria; and an output portconfigured to supply the selected one of the alternate combinations ofthe at least two OFDM signals to a power amplifier, modulated at a radiofrequency, for transmission through an antenna.
 2. The radio transmitteraccording to claim 1, wherein the at least two alternate combinations ofthe at least two OFDM signals vary with respect to a relative timedisplacement of the at least two OFDM signals.
 3. The radio transmitteraccording to claim 2, wherein the at least one selection criteriacomprises a peak level.
 4. The radio transmitter according to claim 2,wherein the at least one selection criteria comprises a peak to averagepower ratio.
 5. The radio transmitter according to claim 2, wherein theat least one selection criteria comprises an estimate of distortion fromthe power amplifier.
 6. The radio transmitter according claim 1, whereinthe OFDM signal communication protocol is compatible with at least oneprotocol selected from the group of an IEEE 802.11 protocol, an IEEE802.16 protocol, a 3GPP-LTE downlink protocol, a DAB protocol and a DVBprotocol.
 7. The radio transmitter according to claim 1, wherein the atleast two alternate combinations of the at least two OFDM signals differrespectively in a cyclic time shift of one OFDM signal with respect toanother OFDM signal.
 8. The radio transmitter according to claim 1,further comprising a computational model of at least the poweramplifier, wherein the selection criteria comprises an evaluation of amodelled performance of at least the power amplifier based on thecomputational model.
 9. The radio transmitter according to claim 1,further comprising a predistorter configured to predistort the selectedone of the alternate combinations of the at least two OFDM signals. 10.A radio frequency transmitter, comprising: an input port configured toreceive at least two orthogonal frequency division multiplexed (OFDM)signals, each having OFDM components within OFDM subchannels of arespective separate OFDM channel, each respective OFDM signal beingcompliant with an OFDM signal communication protocol, such that an OFDMsignal communication protocol compliant receiver can demodulate therespective OFDM signal; a processor configured to define at least twoalternate combinations of the at least two OFDM signals occupying theseparate OFDM channels, each alternate combination having the OFDMcomponents within the respective OFDM subchannels of the respectiveseparate OFDM channels, each of the at least two OFDM signals withineach of the at least two alternate combinations of the at least two OFDMsignals being compliant with the OFDM signal communication protocol,such that the OFDM signal communication protocol compliant receiver candemodulate the respective OFDM signals of each respective alternatecombination without side information outside of the OFDM signalcommunication protocol, and to select one of the alternate combinationsof the at least two OFDM signals based on at least one selectioncriteria; and an output port configured to communicate the selected oneof the alternate combinations of the at least two OFDM signals.
 11. Theradio frequency transmitter according to claim 10, further comprising apower amplifier subject to non-linear distortion of combinations of OFDMsignals, configured to receive the selected one of the alternatecombinations of the at least two OFDM signals from the output port. 12.The radio frequency transmitter according to claim 11, wherein the atleast one selection criteria comprises an estimate of distortion fromthe power amplifier.
 13. The radio frequency transmitter according toclaim 11, further comprising a computational model of at least the poweramplifier, wherein the selection criteria comprises an evaluation of amodelled performance of at least the power amplifier based on thecomputational model.
 14. The radio frequency transmitter according toclaim 10, further comprising a predistorter configured to predistort theselected one of the alternate combinations of the at least two OFDMsignals.
 15. The radio frequency transmitter according to claim 10,wherein the at least two alternate combinations of the at least two OFDMsignals vary with respect to a relative time displacement of the atleast two OFDM signals.
 16. The radio frequency transmitter according toclaim 10, wherein the at least two alternate combinations of the atleast two OFDM signals vary with respect to a relative cyclic time shiftbetween the at least two OFDM signals.
 17. The radio frequencytransmitter according to claim 10, wherein the at least one selectioncriteria comprises a peak level of the alternate combinations of the atleast two OFDM signals.
 18. The radio frequency transmitter according toclaim 10, wherein the at least one selection criteria comprises a peakto average power ratio of the alternate combinations of the at least twoOFDM signals.
 19. The radio frequency transmitter according claim 10,wherein the OFDM signal communication protocol is compatible with atleast one protocol selected from the group of an IEEE 802.11 protocol,an IEEE 802.16 protocol, a 3GPP-LTE downlink protocol, a DAB protocoland a DVB protocol.
 20. A method of transmitting a combined signal,comprising: receiving at least two orthogonal frequency divisionmultiplexed (OFDM) signals, each having OFDM components within OFDMsubchannels of a respective separate OFDM channel, each respective OFDMsignal being compliant with an OFDM signal communication protocol, suchthat an OFDM signal communication protocol compliant receiver candemodulate the respective OFDM signal; defining at least two alternatecombinations of the at least two OFDM signals occupying the separateOFDM channels varying from each other by an amount of a relative timeshift of the respective OFDM signals, each alternate combination havingthe OFDM components within the respective OFDM subchannels of therespective separate OFDM channels, each of the at least two OFDM signalswithin each of the at least two alternate combinations of the at leasttwo OFDM signals being compliant with the OFDM signal communicationprotocol, such that the OFDM signal communication protocol compliantreceiver can demodulate the respective OFDM signals of each respectivealternate combination without side information outside of the OFDMsignal communication protocol; selecting one of the alternatecombinations of the at least two OFDM signals based on at least oneselection criteria; and communicating the selected one of the alternatecombinations of the at least two OFDM signals.